A styrenic resin can employ various types of molding such as injection molding, oriented sheet molding, film molding, foamed sheet molding, foamed board molding, and blow molding, and is excellent in material performance such as transparency, rigidity, and dimensional stability. Furthermore, many styrenic resins can be produced in large quantities at a low price by means of block polymerization by a free-radical polymerization method; solution polymerization with a high concentration of monomers; suspension polymerization; or emulsion polymerization. Accordingly, styrenic resins have been used for a very wide variety of uses. Among them, a homopolymer of styrene, such as polystyrene or GPPS, is a resin that is most versatilely utilized.
Polystyrene serves many uses, due to its many excellent performance characteristics and its cheapness. However, there have been uses unsatisfied even with the performance characteristics of this resin, for example, a use for which the resin is not available due to its poor heat resistance. Specifically, GPPS is about 100° C. (which is its glass transition point) in heat resistance, and thus in any of its uses wherein the resin is brought into contact with heated water-vapor for boiling sterilization, its uses wherein the resin needs to be heated with an electronic oven for food packaging, its uses wherein a molded part of the resin as installed in a vehicle tends to be subjected to a high-temperature atmosphere in summer, and the like, the resultant molded product could not be utilized without the concern of the risk of deformation.
One of the methods of enhancing the heat resistance of polystyrene is a method of copolymerizing styrene and a monomer having a polar functional group, whereby for example, a copolymer (SMAA) of styrene and methacrylic acid, a copolymer (SMA) of styrene and maleic anhydride, a copolymer of styrene and maleimide anhydride, an the like can be derived. The heat resistance of each of these copolymers can be freely changed by controlling the amount of a monomer having a polar functional group incorporated into the copolymer. However, when subjected to a high temperature, a copolymer having a polar functional group may cause the crosslinking reaction of a polymeric chain due to a side reaction of the polar group, followed by the production of a gel-like material, and the degradation of the molding-processing properties due to an increment in viscosity. Thus, such copolymers have not been sufficiently accepted by users in light of quality and productivity considerations.
Additionally, the fact that a copolymer having a polar functional group tends to cause a crosslinking reaction during a high-temperature melt-retention means that the high-molecular-weight product is easily denatured during molding processing. This means that it is difficult to recycle or reuse the resin. For example, when an injection-molded product is derived, a mold sprue and/or a runner may be caused, and when a molded product is derived from a twin-screw stretched sheet or a foamed sheet, a discard (or a skeleton) in addition to the molded product may be formed. Generally, these have been partially mixed with virgin pellets after crushing or cutting so as to be reused, or otherwise partially mixed with a general-purpose resin such as polystyrene to be reused.
However, when the fluidity of the resin is changed due to the crosslinking of the high-molecular-weight product during melt-processing, it is difficult to reuse the resin, and utilization of the resin as a recycling material for virgin pellets may be limited. Furthermore, a copolymer having a polar functional group is generally incompatible with polystyrene, and even when melted and mixed with polystyrene, not only the mechanical properties are decreased, but also the transparency is lost. Due to such problems, a copolymer having a polar functional group has not been utilized even as a recycling material for a general-purpose polystyrene.
In recent years, a high value has been attached to the effective utilization of a resin, while various recycle laws were instituted and have been enforced. It may be required in a commercial scene of future that a resin can be recycled, reworked and reused. As a resin material to be developed in the future, a resin which is reused effectively without substantially causing the generation of a monomer or a decrease in molecular weight due to the breakage of high-molecular chains, even if the resin material is subjected to melt processing several times, will be needed. Therefore, the development of such a resin material which is higher in melt stability than a conventional styrenic copolymer has been desired.
Another problem of a conventional styrenic resin having heat resistance is that the range of working conditions during molding is narrow.
A copolymer being enhanced in heat resistance is synonymous with enhancement of the temperature at which a polymer chain starts to be fluidized. Therefore, in order to derive the same fluidity as that of polystyrene during molding processing, a processing temperature should be enhanced corresponding to the increase of heat resistance. However, a kick-off temperature of a styrenic copolymer having a polar functional group can not be enhanced corresponding to the increase of heat resistance. Therefore, there has been the problem that the range of molding processing temperature is narrowed, whereby its productivity and quality are decreased.
There is also a method for enhancing the heat resistance of a styrenic resin by using a monomer having no polar functional group. For example, it is known that a copolymer of styrene and α-methylstyrene has its glass-transition temperature enhanced according to the content of α-methylstyrene (see, for example, Non-Patent Document No. 1). However, α-methylstyrene has a ceiling temperature as low as about 60° C. Therefore, the copolymerization of styrene and α-methylstyrene was attempted using a radical solution-polymerization, which is a typical example of the industrial production method. As a result, a number of problems were detected, for example, 1) that it is difficult to derive a high-molecular-weight product; 2) that the content of α-methylstyrene in the copolymer is limited, whereby a desired heat resistance cannot be achieved; 3) that because of bad heat-stability during melting, some molding-processing conditions cause thermal decomposition of the copolymer, whereby the generation of a monomer component and/or a decrease in molecular weight tend to be caused; 4) that the resin pellets tend to be yellowed, and thus some uses need the addition of a coloring agent; and the like. Therefore, no copolymers of styrene and α-methylstyrene have ever been industrially utilized.
On the other hand, since α-methylstyrene can be subjected to a polymerization by living anionic polymerization with a butyllithium initiator, a copolymer of styrene and α-methylstyrene can be produced by living anionic polymerization (see, for example, Patent Document No. 1).
However, with respect to a copolymer as derived on the basis of a known production method by living anionic polymerization, the following problems have been detected. Therefore, sufficient usefulness as a resin product could not be found therefrom, and thus no such copolymers have ever been industrially utilized.
That is, the above problems are as follows:                1) A produced polymer will be yellowed. The degree of yellowing is correlated with the content of Li. Therefore, there was an area wherein the balance between a molecular weight objective and yellowing property is upset, so that it was difficult to utilize the polymer, in particular, for uses wherein yellowing is not preferred, such as a use for a food packaging, and a use for an optical product.        2) The polymer shows bad heat-stability during melting, and thus the polymer is decomposed during its melt retention so as to generate styrene and α-methylstyrene. The amount of the decomposition products is larger than the amount of the decomposition products from polystyrene as produced according to a free-radical polymerization method which is generally and widely utilized, under the same conditions. This fact means that when the molding-processing temperature of the copolymer of styrene and α-methylstyrene is elevated by the degree to which the heat resistance of the copolymer is higher enhanced than that of polystyrene, the copolymer generates styrene and α-methylstyrene during molding in a larger amount than the amount of the decomposition products from polystyrene as derived according to free-radical polymerization method. Therefore, it is easily predicted that there will occur the problems that some molding conditions tend to cause a silver blister due to volatile components as generated by the decomposition, such as styrene, α-methylstyrene and the like; that the molecular weight of the copolymer tends to be decreased, which is prone to cause the deterioration of the mechanical properties; in particular, that it is difficult to reuse the resultant molded product as a recycling material; and the like. The fact that the molding processing can be utilized merely in an extremely limited range naturally means that the practical uses of the copolymer are limited. Therefore, it can be presumed that such polymers have not been widely and industrially accepted.        
As another disadvantage of polystyrene, the disadvantage that polystyrene is not suitable for a use wherein polystyrene may be exposed to the sun, due to its bad weatherability, can be enumerated. The bad weatherability is predominantly due to the structure of the high-molecular-weight product. Therefore, first of all, the development of a styrenic copolymer, having the weatherability of the high-molecular-weight product itself enhanced without relying on an additive such as a weathering stabilizer or an ultraviolet absorber, has been desired.
Non-Patent Document No. 1: Journal of Applied Polymer Science, Vol. 41, p. 383 (1990).
Patent Document No. 1: Japanese Patent KOKOKU Publication (JPB) No. 6-10219.